A heat pump type air conditioner comprises a refrigerating cycle including a compressor 1, a four-way valve 2, an indoor heat exchanger 3, an expansion valve (an electronic expansion valve) 5, and an outdoor heat exchanger 4, as schematically shown in FIG. 5.
In addition, to defrost the outdoor heat exchanger 4 during a heating operation, a bypass pipe 7 having a solenoid-operated valve 6 is provided between a refrigerant ejection port of the compressor 1 and a refrigerant inflow port of the outdoor heat exchanger 4. The solenoid-operated valve 6 is normally "closed."
During a heating operation, the refrigerant is circulated from the compressor 1 through the four-way valve 2, the indoor heat exchanger 3, electronic expansion valve 5, outdoor heat exchanger 4, and four-way valve 2 to the compressor 1, as shown by the continuous-line arrow in FIG. 5.
During this heating operation, the indoor machine side having the indoor heat exchanger 3 rotationally controls an indoor fan to blow into the room, warm air obtained by a heat exchanging operation executed by the indoor heat exchanger 3, and transfers to the outdoor machine side an operational-frequency code corresponding to the difference between the indoor temperature and a set temperature on a remote controller.
The outdoor machine side having the outdoor heat exchanger 4 operates the compressor 1 according to the operational-frequency code to circulate the refrigerant. This operation controls the room temperature to the set temperature by a remote controller.
Specifically, the indoor and outdoor machines each include a control section consisting of a microcomputer. The control section on the indoor machine side controls the indoor fan according to an instruction from the remote controller and transfers to the control section on the outdoor machine side, data such as the operational frequency corresponding to the difference between the room temperature and the set value. The control section on the outdoor machine side controls the compressor and the outdoor fan according to the data.
If the outdoor heat exchanger 4 is frosted, its heat exchanging efficiency decreases to degrade the performance of the air conditioner, so the outdoor heat exchanger 4 must be defrosted.
The defrosting method includes a hot gas bypass defrosting method and a reverse defrosting method. Either of these methods is conventionally used.
In the hot gas bypass defrosting method, when the temperature of the outdoor heat exchanger 4 decreases down to a defrosting start value (for example, -10.degree. C.; see FIG. 6), the control section determines that the heat exchanger is frosted to open the solenoid-operated valve 6 in order to supply part of a refrigerant (a hot gas) ejected from the compressor 1 to the outdoor heat exchanger 4 via the bypass pipe 7 (see the continuous-line arrow in FIG. 5).
Thus, the outdoor heat exchanger 4 is defrosted. When the temperature of the outdoor heat exchanger 4 reaches a defrost cancel value, the solenoid-operated valve 6 is closed to return to the normal heating operation.
The hot gas bypass defrosting method allows the refrigerant circulating path for the heating operation to be used during the refrigerating cycle without change and enables the outdoor heat exchanger 4 to be defrosted while continuing the heating operation.
In the reverse defrosting method, when the temperature of the outdoor heat exchanger 4 decreases down to a defrosting start value, the four-way valve 2 is switched to reverse the flow of the refrigerant (see the chain line arrow in FIG. 5) in order to feed a hot gas into the outdoor heat exchanger 4. During this defrosting, the indoor fan on the indoor machine side is stopped. When the temperature of the outdoor heat exchanger 4 reaches the defrost cancel value, the four-way valve 2 is switched again to return the flow of the refrigerant to the original direction.
Thus, the reverse defrosting method enables the outdoor heat exchanger 4 to be defrosted in a short time and does not require the bypass pipe 7 or solenoid-operated valve 6, thereby precluding the costs of the air conditioner from increasing.
Both methods, however, have both advantages and disadvantages. The reverse defrosting method enables defrosting to be executed in a relatively short time despite a large amount of frost, but is disadvantageous in that the temporary interruption of the heating operation causes the room temperature to decrease during this interruption.
On the contrary, the hot gas bypass defrosting method only reduces the amount of refrigerant circulated to the indoor heat exchanger 3, so it does not significantly reduce the room temperature. This method, however, is disadvantageous in that since the temperature of the outdoor heat exchanger 4 increases slowly as shown in the temperature graph of the outdoor heat exchanger 4 in FIG. 6, a large amount of time is required for the defrost cancel temperature to be reached.